#include <algorithm>
#include <bits/stdc++.h>
using namespace std;

#define int int64_t
#define endl "\n"

using PII = pair<int, int>;
using TII = tuple<int, int, int>;
template<class T> using V = vector<T>;
using i64 = long long;
using i128 = __int128;
using u64 = unsigned long long;

#define FOR(i, a, b) for (int i = (int)(a); i <= (int)(b); i++)
#define FOR2(i, a, b, c) for (int i = (int)(a); i <= (int)(b); i += c)
#define FORD(i, b, a) for (int i = (int)(a); i >= (int)(b); i--)
#define FORD2(i, b, a, c) for (int i = (int)(a); i >= (int)(b); i -= c)
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define lowbit(x) ((x) & (-x))
#define RE return;
#define Yes cout << "Yes" << endl;
#define YES cout << "YES" << endl;
#define No cout << "No" << endl;
#define NO cout << "NO" << endl;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define sz(x) (int)(x).size()

void solve() {
    string a, b;
    cin >> a >> b;

    int j = sz(b) - 1;

    V<int> ans;

    FORD(i, 0, sz(a) - 1) {
        if (j < 0) {
            cout << -1 << endl;
            RE;
        }
        if (b[j] >= a[i]) {
            ans.eb(b[j] - a[i]);
            j--;
        } else {
            if (j - 1 < 0) {
                cout << -1 << endl;
                RE;
            }
            if (b[j - 1] >= '2') {
                cout << -1 << endl;
                RE;
            } else {
                ans.eb((b[j - 1] - '0') * 10 + (b[j] - '0') - (a[i] - '0'));
                j -= 2;
            }
        }
    }
    while (j != -1) {
        ans.eb(b[j] - '0');
        j--;
    }
    while (sz(ans) > 1 and ans.back() == 0) ans.pop_back();
    reverse(ALL(ans));
    for (auto x : ans) {
        if (x < 0) {
            cout << -1 << endl;
            RE;
        }
    }
    for (auto x : ans) cout << x;
    cout << endl;
}

signed main() {
    int Task = 1;

    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> Task;
    while (Task--) {
        solve();
    }


    return 0;
}
